This application claims the priority of Korean Patent Application No. 2003-96220, filed on Dec. 24, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a method of fabricating an organic thin film transistor, and more particularly, to a method of fabricating a bottom gate type organic thin film transistor in which a gate conductive layer pattern is disposed below an organic semiconductor thin film pattern.
2. Description of the Related Art
Currently, inorganic thin film devices are being rapidly replaced by organic thin film devices due to improvements in technology and active research of organic thin film devices. Research and development of organic thin film transistors for realizing organic semiconductor devices has been the main reason for the change. An organic thin film transistor includes an organic semiconductor thin film as an active layer, and a gate conductive layer pattern for controlling the conductivity of an organic semiconductor thin film. A gate insulating layer is interposed between the organic semiconductor thin film and the gate conductive layer pattern, and source/drain electrodes are used as a path for an electric charge.
Organic thin film transistors are categorized into two types according to configuration. One is a top gate type in which a gate conductive layer pattern is formed on an organic semiconductor thin film pattern. The other is a bottom gate type in which a gate conductive layer pattern is formed on under an organic semiconductor thin film pattern. Bottom gate type organic thin film transistors are categorized into two types depending on the order of deposition used. One has a configuration in which a gate conductive layer pattern, a gate insulating layer, source/drain electrodes, and an organic semiconductor thin film pattern are deposited sequentially. The other has a configuration in which a gate conductive layer pattern, a gate insulating layer, an organic semiconductor thin film pattern, and source/drain electrodes are deposited sequentially.
To fabricate the top gate type organic thin film transistor, an organic semiconductor thin film pattern is formed and then a gate insulating layer is formed on the organic semiconductor thin film pattern. Forming a gate insulating layer having an excellent feature generally requires by a high-temperature process. However, generally, an organic semiconductor thin film pattern is not stable at a temperature over about 150° C. Thus, certain features can be deteriorated in an organic semiconductor thin film pattern during a high-temperature process for forming a good quality gate insulating layer. And since the surface morphology of the organic semiconductor is not good, the interface between organic semiconductor and gate insulator as a channel is not good and the characteristics of organic thin film transistor can be deteriorated. For these reasons, a bottom gate type organic thin film transistor is used more often than a top gate type organic thin film transistor.
But there are also several problems in fabricating a bottom gate type organic thin film transistor, too. For example, to form an organic semiconductor thin film pattern, an organic semiconductor thin film should be formed, and then the organic thin film should be patterned. Generally, a patterning process may be performed by forming a mask layer pattern on an object layer and etching the object layer using the mask layer pattern as an etch mask. The mask layer pattern is generally of a photoresist layer pattern. When forming a photoresist layer pattern on an organic semiconductor thin film and patterning the semiconductor thin film using the photoresist layer pattern, a solvent peels the organic semiconductor thin film off and rapidly deteriorates the characteristics of the organic semiconductor thin film. Thus, forming source/drain electrodes on an organic semiconductor thin film pattern becomes much more difficult.
A configuration in which an organic semiconductor thin film pattern is deposited on a gate insulating layer and source/drain electrodes are deposited on the organic semiconductor thin film pattern tends to have the best characteristics. When the organic semiconductor thin film pattern is formed on the gate insulating layer, grains in the organic semiconductor thin film pattern become larger, and thus a crystalline property is improved, thereby increasing the mobility of electrons or holes. When forming the source/drain electrodes on the organic semiconductor thin film pattern, a contact resistance formed between the source/drain electrodes and the organic semiconductor thin film pattern decrease, thereby improving the operation of an element.